Heavy oil production by thermal methods



G. D. GOULD HEAVY OIL PRODUCTION BY THERMAL METHODS Filed Jan. 4, 1960 Nov. 20, 1962 United States Patent Olriice 3,@64J28 Patented Nov. Z0, 1952 3,064,723 MAW Gili PRQDUCTIGN BY THERE/ieri. METHDS George D. Gould, rinda, Salif., assigner to talifornia Research Corporation, San Francisco, flaiif., a corpo= ration of Deiaware Filed Een. 4, 195), Ser. No. i5

i Ciaims. (Cl. loo-n2):

The present invention relates to assisted oil recovery methods. More particularly, it relates to a method of recovering heavy oil by developing a residual heat bank in the formation, injecting oil-soluble gas into the formation to set up a zone of reduced viscosity oil at a place removed from the heat bank, flowing a liquid through the residual heat bank toward the oil-gas solution zone,

and alternately recovering the reduced viscosity oil from the zone and reinjecting oil-soluble gas into the zone to lower the viscosity of the new oil moving into the zone.

lt is an object of the present invention to increase the production of oil from an oil-bearing formation wherein `the oil viscosity is so relatively high that it does not satisfactorily lend itself to recovery by more conventional methods. The method of the invention is particularly Well adapted to the recovery of very heavy crudes such as are commonly found in the California and Pennsylvania elds. Although the term heavy oil7 is only relative nomenclature and may be deiined differently in dif- `ferent localities, the method of this invention would be the same in all areas and does not depend on local definition. It is contemplated that the process would have its greatest adaptation in recovering oils with a gravity of less than 20 API. The method of this invention is not limited to secondary recovery. It may be utilized with equal eectiveness in either primary or secondary recovery.

Methods previously tried with the hope of accomplishing the economical recovery of highly viscous oils have included improved pumping practices, such as the longs'troke method of pumping, the air and gas lift techniques, and many methods of assisted recovery drives employing either water, gas, underground combustion, or a like driver. These methods have all been relatively unsuccessful in formations containing very viscous oilsince noneof them are directed to both reducing the oils viscosity, at least `in the vicinity of the producing Well so that the oil can be more readily moved into the Well, and simultaneously applying pressure by injection of a driving fluid in a neighboring well so that a pressure differential is set up between the wells. In ordinary assisted recovery methods, oil is moved toward and into a producing well by pressure applied by means of an injected driving uid. However, in the case of highly viscous oils referred to here, in order to move the oil at a desirable rate, excessively high pressure is required. For the above reasons, it is necessary not only to drive the oil toward the producing well by external pressure applied to the formation but also to enhance the mobility of the oil around the producing well -by viscosity reduction in place.

Additional objects and advantages of the present invention will become apparent from the following detailed description read in light of the accompanying drawing which is made a part of this specification and in which:

FIG. l is a schematic representation of a vertical section of an oil-bearing formation being produced by the method of the present invention.

Referring speciii'cally to FlG. l, an oil-bearing formation 21 penetrated by an injection well 22 and a production well 24 is shown. In accordance with the method of the invention a heated zone 26 is developed in for- 2 mation 2l. This may be accomplished by igniting the oil in formation 2l and injecting an oxygen-containing gas from source 28 through injection well 22 into formation 2l to support combustion therein. After moving cornbustion out into formation 2l the injection of oxygencontaining gas is stopped and combustion extinguished.

Oil-soluble gas from source 30 is injected through pipe 3l in production well 24 into formation 21. The oilsoluble gas goes into solution with the oil in the vicinity of production well 24 to establish a zone of reduced viscosity oil 32.

Water is injected from' a suitable Water source through pipe 35 in injection well 22 into formation 21 and heated zone 26. Alternately during the water injection oil is recovered from formation 2l through production Well 24 by means of pump 33 and pipe 37 and, as the oil viscosity increases, oilsoluble gas is injected through pipe 31 in production well 2.4 into formation 21 to again reduce the oils viscosity.

In accordance with the preferred method of carrying out the present invention a heated Zone rst is established in the heavy oil formation. One method of creating the heated Zone is by underground combustion. The ignition of the formation may be accomplished by a downhole burner, chemicals capable of producing exothermic reaction, or by spontaneous ignition of the formation. Once the combustion front is established in the formation, it is moved through the formation a predetermined distance by the injection of air or oxygen through the Well wherein ignition was accomplished. After Vthe combustion front has traveled a desired distance into the formation from the ignition hole, the supply of oxygen-containing gas is discontinued and the combustion is extinguished. An oil-soluble gas is then injected through one or more other Wells which intersect the producing formation at points away `from the area of combustion. The oil-soluble gas is injected under sufficient pressure so that it is forced into the oil-bearing formation surrounding these gas-injection wells. The pressure of injection is such as to maximize the amount of gas dissolving in the heavy oil in the zone around the point of injection. The gas may be injected under an initial high pressure or may be initially injected at a relatively low pressure with the pressure thereafter being gradually increased until the optimum pressure for solution is reached. The optimum injection pressure is that pressure at which the maximum amount of gas will dissolve in the oil. The amount of gas that will go into solution varies directly with the pressure on the system and the viscosity of the heavy oil varies inversely with the amount of gas in solution. Therefore, injecting oil-soluble gas into the producing formation under pressure so that it dissolves in the oil will set up a zone of relatively less viscous oil which may be more easily removed to the produ-cing well, there being better handled by the pumping equipment. Gases which are soluble in oil and which could be used as the injection gas include methane, ethane, propane, butane, isobutane, ethylene, propylene, acetylene, hydrogen sulfide, carbon dioxide, and mixtures of these gases, such as natural gases. The gases used in this method would generally be found in a natural state in the area and therefore they will be under some pressure. However, if this pressure is not sufficient to effect solution, a compressor is used to raise the gas pressure to the desired pressure. The gas injection well is now held under a back pressure which is suiiicient to hold the `gas-in solution with the formation oil. After the gas injection has been completed, a Water injection drive is initiated through the distant borehole wherein the combustion front had been initiated and moved into the formation. This water drive moves through the residual hea't bank which remains in the formation after the combustion front had been extinguished. The driving fluid picks up heat from the residual heat bank and creates a driving front of hot oil, steam, and hot water, which accomplishes lluid displacement which in turn exerts a pressure on the oil in formation, causing it to move in the direction of the zone of relatively less viscous oil resulting from the oil-gas solution. The gas-injection well is now converted to an oil production well. Oil displaced by -the advancing thermal front exerts pressure on the zone of relatively less Viscous oil, and the less viscous oil plus dissolved gas is produced through the gas injection-oil production well until the produced oil viscosity eventually approaches the original viscosity of the oil in the formation. As the original higher oil viscosity is approached, a point will be reached wherein the production rate diminishes and it is no longer economical to recover the viscous product. At this time oil production is stopped and more oil-soluble gas is again injected through the gas injection-oil production well. During the reinjection of oil-soluble gas, the uid drive through the residual heat bank is kept static. After a predetermined amount of oil-soluble gas has been injected, the thermal drive is reinitiated and oil production is again accomplished by returning the gas injection Well to oil production under back pressure. Thus, a cyclic process is set up wherein oil-soluble gas is injected through the gas injection-oil production Well until a desired viscosity reduction has taken place in the zone near this Well. Then water is injected through the residual heat bank causing uid flow toward the gas injection-oil production Well and the less viscous oil is produced through this Well until the viscosity again reaches the point where economics dictate the injection of more oil-soluble gas, and the cycle begins again.

In some applications of the invention, where the assisted recovery drive is expected to be active over extended periods of time, it may be necessary to re-establish the heat bank. This may be accomplished While the water injection has been stopped to allow gas injection cycle to be completed but before the gas injection has begun. The preferred method of re-establishing the heat bank is to reignite the formation at the Water injection well and to support the combustion with air injection until the combustion front has proceeded a predetermined distance into the formation. Combustion is then extinguished and the Water drive through the revitalized heat bank may be again initaited. An alternative method of re-establishing the heat bank comprises injecting hot water or steam throughthe water injection well for a period long enough to set up a residual heat bank through which the uid drive can be reinitiated. Y

In selecting the driving force for the assisted recovery drive, it is important that the fluid utilized Will not strip 'the gas Vfrom thel oil-gas solution. The injected fluid, therefore, must be one in which the gas is less soluble than the oil. Water is the preferred injection fluid for the assisted recovery drive. The method of the invention would not allow the continuation of a combustion drive during the injection of gas and production of oil at the gas injection-oil production well because the prod- -ucts of combustion would tend to strip the gas from the oil-gas solution. If this stripping were to occur the advantage gained by the viscosity reduction due to the so- `1ution of gas would obviously be lost.

. A typical application of the invention in a two-well system wherein well No. 1 is the combustion-water in- `jection well andrwell No. 2 is the gas injection-oil recovery well is'as follows. Ignition of the oil in formation -by'one of the methods described is first accomplished through well No. 1. This combustion is moved a prede- `termined distance into the formation by the injection of air through well No. 1. After a predetermined time, the supply of air is discontinued and combustion thereupon ."Ceases, leaving a residual heat bank in the formation.

Immediately thereafter a predetermined amount of oilsoluble gas is injected into the formation through well No. 2. After the injection of oil-soluble gas through well No. 2 is completed, water is injected through well No. 1 into the residual heat bank. As this Water injection continues, a front of hot oil, hot water and steam moves toward well No. 2, thus exerting pressure on the zone of less viscous oil around Well No. 2. Oil and gas are now produced at well No. 2 until the oil viscosity reaches a point where, because of the increased cost and diiculty of pumping the viscous oil, economics dictate that gas be reinjected into the formation to reduce the viscosity of the oil contained therein. When this point is reached, the gas injection is again initiated at well No. 2 and the injection of water is discontinued through well No. 1. After a predetermined amount of gas has been injected through well No. 2, the water is again introduced through well No. 1 and oil and dissolved gas are again produced at Well No. 2 under backpressure.

It will be apparent to those skilled in the art that the present system permits an appreciable increase in the eliiciency of the assisted recovery from the relatively heavy oil bearing formations. Various changes in the number and pattern of the Well bores and in the method of injecting and recovering the oil will be obvious to those skilled in the art. All such modifications or changes falling within the scope of the appended claims are intended to be included herein.

I claim:

t1. A method for assisting the recovery of oil from a petroleum-bearing formation penetrated by at least one injection well and one recovery well comprising igniting a portion of the oil in the vicinity of said injection well, supplying an oxygen-containing gas through said injection well to move combustion a predetermined distance into said formation to raise the temperature of a portion of said formation, stopping said supply of oxygen-containing gas, injecting an oil-soluble gas into said formation through said recovery well to reduce the viscosity of the oil in a zone around said well, holding a back pressure on said recovery well sufficient to keep gas in solution with the formation oil, injecting a fluid through -said injection well into said raised temperature portion of said formation to drive oil toward said recovery Well, and alternately recovering oil from said recovery well and injecting oil-soluble gas through said recovery well into said formation.

2. v A method of recovering oil from an oil-bearing formation penetrated by at least one injection well and one production well comprising the steps of igniting a portion of the oil in said formation in the vicinity of said injection well to establish a combustion zone inV said formation, injecting an oxygen-containing gas through said injection well to move said combustion zone out into'said formation, discontinuing injection of said oxygen-containing gas to extinguish said combustion, injecting an oil-soluble gas into said formation throughsaid production well to establish a zone of less viscous oil in said formation, holding the zone of less viscous oil under suicient back pressure to retain gas in solution with the oil, forcing water into said formation through said injection well and alternately recovering oil under back pressure through said production well from said zone of less viscous oil and reinjecting oil-soluble .gas through said production well into said formation.

3. Inra system comprising a plurality of wells all of Vwhich penetrate an oil-bea1ing formation and which include at least an injection well and a production Well, the

method of recovering oil from said formation comprising the steps of igniting a portion of the oil in said formation in the vicinity of said injection well to establish combustion in said formation, injecting an oxygen-containing gas through said injection well to move said combustion away from said injection well into said formation, discontinuing the injection of said oxygen-containing gas .MAM

through said injection Well, injecting an oil-soluble gas through said production well to establish a zone of relatively less viscous oil in said formation, holding said production well under back pressure sufficient to cause said oil-soluble gas to remain in solution with the oil in said formation, forcing water through said injection well into the combusted zone of said formation, and alternately recovering the less viscous oil through said production well and, when the viscosity of the oil approaches the original viscosity, injecting oil-soluble gas through said production well into said formation to again reduce the viscosity of the oil in said formation in the vicinity of said production well for continued production therefrom.

4. In a system comprising at least two wells, both of which penetrate an oil-bearing formation, the method of assisting the recovery of oil comprising the steps of igniting the oil in said formation through well No. 1, moving combustion a predetermined distance into said formation by injecting combustion-supporting gas through well No. 1, stopping said injection to extinguish said combustion thereby forming a residual heat bank, injecting a predetermined amount of oil-soluble gas into said formation through well No. 2 to reduce the viscosity of the oil in a zone around well No. 2, holding well No. 2 under a back pressure suicient to keep said oil-soluble gas in solution with the oil, injecting a uid through Well No. 1 into the residual heat bank thereby causing a pressure increase on the formation at well No. 2, and recovering oil through Well No. 2 until the viscosity of the oil-gas solution approaches the original viscosity of the oil, and then reinjecting oil-soluble gas into said formation.

Doherty Feb. 28, 1933 Pelzer Apr. 9, 1957 

1. A METHOD FOR ASSISTING THE RECOVERY OF OIL FROM A PETROLEUM-BEARING FORMATION PENETRATED BY AT LEAST ONE INJECTION WELL AND ONE RECOVERY WELL COMPRISING IGNITING 